1. Field of the Invention
The invention relates to a method for operating a microlithographic projection exposure apparatus designed for wavelengths in the extreme ultraviolet spectral range (EUV). The invention furthermore relates to a microlithographic projection exposure apparatus suitable for carrying out the method.
2. Description of the Prior Art
Microlithographic projection exposure apparatuses are used to transfer structures contained in a mask or arranged thereon to a photoresist or some other light-sensitive layer. The most important optical components of a projection exposure apparatus are a light source, an illuminating system, which conditions the projection light generated by the light source and directs it onto the mask, and a projection objective, which images that region of the mask which is illuminated by the illumination system onto the light-sensitive layer.
The shorter the wavelength of the projection light, the smaller the structures that can be produced on the light-sensitive layer with the aid of the projection exposure apparatus. The most recent generation of projection exposure apparatuses uses projection light having a center wavelength of approximately 13.5 nm, which is therefore in the extreme ultraviolet spectral range (EUV). Such apparatuses are often referred to as EUV projection exposure apparatuses.
However, there are no optical materials which have a sufficiently high transmissivity for such short wavelengths. Therefore, in EUV projection exposure apparatuses the lenses and other refractive optical elements that are customary at longer wavelengths are replaced by mirrors, and the mask, too, therefore contains a pattern of reflective structures.
The provision of mirrors for EUV projection exposure apparatuses constitutes a major technological challenge. Coatings which are suitable for EUV light and are applied to a mirror substrate often comprise more than 30 or 40 double layers having a thickness of just a few nanometers, which are vapor-deposited one above another in technologically complex processes. Even with coatings of such complex construction, the reflectivity of the mirrors for the EUV light is usually hardly more than 70%, and even this applies only to light that impinges on the reflective coating perpendicularly or with angles of incidence of a few degrees.
The comparatively low reflectivity of the mirrors has the consequence that in the development of EUV projection exposure apparatuses efforts have to be made to use as few mirrors as possible, since each mirror involves light losses and ultimately reduces the throughput of the projection exposure apparatus.
For the illumination system of such apparatuses this means, inter alia, that it is not possible to use an optical unit to image an adjustable field stop onto the mask, as is the case with illumination systems for longer wavelengths around the DUV or VUV spectral range. Such adjustable field stops contain moveable stop elements, which are occasionally also designated as reticle masking blades. Owing to an imaging optical unit being dispensed with in this way, in EUV projection exposure apparatuses the field stop is arranged as near as possible to the mask.
However, an arrangement of the field stop in direct proximity to the mask is associated with a number of disadvantages. In this regard, owing to the small but nevertheless finite distance between the field stop and the mask, it turns out that the edges of the illumination field illuminated on the mask are not sharp. Moreover, an (albeit small) part of the light reflected and diffracted at the mask is shaded by the field stop.
A further problem that arises as a result of the arrangement of the field stop in direct proximity to the mask is associated with the fact that the reticle masking blades are moved at high speed and high acceleration during the scanning process. The reticle masking blades ensure that the dimensions of the illumination field along the scanning direction are continuously increased at the beginning of the scanning process and are reduced again at the end of the scanning process. The rapid and frequent travel movements of the reticle masking blades foster the production of tiny abraded particles. However, since the abraded particles are released in direct proximity to the mask, they can be imaged onto the light-sensitive layer, which can lead to defects in the lithographically produced components.